Electron lens assembly having electrodes with toroidal terminations



March 14, 1967 A. D. GIACCHETTI ETAL 3,309,559

ELECTRON LENS ASSEMBLY HAVING ELECTRODES L WITH TOROIDAL TERMINATIONS Filed March 18, 1964 APPQOX. 20/116 70 27/54 APPEUX. KAIVG IG. 6 FIG, 6 F166 121.4 VJJIIIJJI I J ifs-J0 Fm. 2%.12 F2615 INVENTO I Gnaclefo D.6zacchc1 c BY Ddllll J. Pinch United States Patent 3,309,559 ELECTRON LENS ASSEMBLY HAVING ELECTRODES WITH TOROIDAL TER- MINATIONS Anacleto D. Giacchetti, Lemont, and Daniel J. Plach,

Riverside, Ill., assignors to National Video Corporation, Chicago, Ill., a corporation of Illinois Filed Mar. 18, 1964, Ser. No. 352,847 2 Claims. (Cl. 315-16) This invention relates to electrodes operative in an evacuated envelope which may assume a potential which is comparatively high compared to an adjacent conductive element and which may, under certain operating conditions, constitute one terminal of an arc discharge or, with reference to gas in the envelope, initiate corona. Since, in nearly every case, arc discharge or corona adversely affects the desired operating characteristics, their elimination or reduction to a tolerable state is a desirable objective.

It is a fundamental principle of electrostatics that the smaller the radius of a conducting surface the greater the intensity of the electric field. For example, the corners of a square cut edge of a simple hollow cylindrical electrode, such as may constitute an accelerating electrode of an electron gun of a kinescope, may be regarded as arcs of minute radius and, therefore, will have a substantially greater field intensity than the principal cylindrical surface. Accordingly, with an electrode of this character, the conditions leading to arcing or corona are greatly aggravated.

It has been known to curl one or both ends of a hollow cylindrical electrode in an attempt to reduce the severity of the problem set forth above but the advantage is illusory rather than real since the square cut end is simply shifted to another location. The fallacy lies in assuming that tucking the square cut end within the curl or juxtaposed to the exterior surface of the cylinder eliminates its ill effects, whereas the fact of the matter is that the electrostatic field will continue to establish its maximum density at the square corners, wherever they may be positioned in a geometrical sense. Bufiing or tumbling of the electrode is of little or no avail as the minuscule amount of material or burrs removed thereby has little effect on the radius of the corners.

The present invention has for its principal object improvements in the construction of electrodes operating in the stated environment whereby sharp corners discontinuities in the configuration of the electrode are eliminated. As mentioned above, in the case of a hollow cylindrical electrode, the problems set forth exist at the sharp corners at the ends thereof and, therefore, the present specification is, exemplificatively, directed to configurations of this class. It will be noted, however, that the term electrode is used herein in ,a much broader scope and, as the exposition proceeds, it will become apparent that other forms of electrodes, such as those which are discs, annuli, etc., may be improved by employing the principles of the invention.

In the development of television picture tubes, for example, the need has arisen for substantial increase in focus and accelerating potentials which, in turn, has aggravated the problems incident upon arcing and corona. Accordingly, another important advantage of the instant 3,309,559 Patented Mar. 14, 1967 concept resides in permitting substantial increase in such potentials. In the case of an electron gun wherein G is in overlapping relation with G and G such increase may range from 8 to 10 kv. In the case of :a gun wherein G is shorter than the axial distance between the confronting ends of G and G the increase may range from 10 to 15 kv. As alluded to above, former electrodes having curled edges show a gain in potential of only 2 to 3 kv.

It has also been found that the overlapping of G and G by G, heretofore deemed necessary to alleviate arcing and corona discharge may be dispensed with, and advantage taken of the higher potentials set forth above. Thus by spacing G, from G, and G considerably improved corner and edge resolution of the raster may be achieved by reason of the more uniform field constituting the focusing lens. Plots of the equipotential planes of the former overlapping electrodes displayed considerable bunching between and about the overlapped ends which, in turn, resulted in deterioration of the corner and edge focus. When the invention improvement is employed, the length of G is capable of being varied over a comparatively wide range, subject to appropriate spacing between G G and G G such as will permit maintenance of the specified focus potential on G in a typical kinescope gun of say between and +400 volts. Furthermore, when the length of G is reduced the area available for cold emission is reduced proportionately, and cold emission correspondingly reduced. However, once the desired length of G is determined, and the operating potential (or range of operating potentials) for G G and G established, such length will need to be held to close tolerances in order to maintain the specified focus voltage. Additionally, the diameter of 6., should be larger than the adjacent portions of G and G to obtain as much clearance as possible between the low potential G and the two high potential electrodes G and G Other objects and advantages of the invention will become evident from the ensuing description which, together with the accompanying drawing, discloses certain preferred forms in which the invention may be embodied in practice.

In this drawing:

FIG. 1 is -a somewhat diagrammatic longitudinal cross section of the neck portion of a kinescope as used for television reception, including one form of conventional electron gun; and

FIGS. 2 to 15 are partial cross sections of an electrode, e.g., the focusing electrode of an electron gun of the type shown in FIG. 1.

Adverting to FIG. 1, the neck 10 of the kinescope surrounds an electron gun 12 fixedly mounted in the tube envelope, comprising the several active elements which are maintained in their operative relation by suitable insulating supports (not shown) and provided with leads and jumpers (not shown), following customary practice. It will be noted that the high voltage for the foremost electrode is furnished through the usual coating of dag 14. The cathode is omitted but will be understood to be of a common type.

The electrodes G and G are the control grid and screen grid, respectively, and, since their function is wellknown and they do not enter into the performance of the focusing electrodes to any significant extent, further description is not deemed essential.

In the exemplificative gun, focusing of the beam is accomplished through the medium of G G and G In this case, G and G are tied together with a jumper and, absent the invention improvement, operate at a potential of from 18,000 to 25,000 volts. The focus electrode G operates at from lO to +400 volts and, influenced as it is by the neighboring high potential, acts as a cathode source for cold emission, ionizing the residual gas to initiate an arc discharge. When G is made as a plain, hollow cylinder, the corners of the end faces, being relatively sharp or of some minute radius as a result of polishing, create a high density of electric field, i.e., high potential, which induces corona and arcing. Research has demonstrated that curling of the material of the cylinder over an are extending from about 180 to 360 is relatively ineffective to preclude arcing or corona since the sharp edge is simply shifted to a position not significantly removed from the adjacent zone of high potential. Moreover, even if the edge of an uncurled cylinder or of a curled end portion is sufficiently polished to a rounded configuration the radius thereof, which can be no greater than one-half the thickness of the cylinder wall, remains of essentially minute radius so that the slight advantage accruing therefrom is not regarded as useful in improving the operation of the gun.

In accordance with the invention, the focusing electrode G is provided with toroidal or substantially toroidal ends either in the form of a ring secured to the ends of a plain, hollow cylinder or by curling the ends thereof and providin g a fillet to mask, so to speak, the electrostatic effect of the edge terminating the curl. In those cases where the toroid meets the wall of the cylinder in a re-entrant angle of 90 more or less, no new problem is introduced since the field distribution at the intersection of two planes at this angle is substantially less dense than at a salient angle of substantially 90.

Thus, referring to FIG. 2, there is shown an electrode 20 constituted as a thin-walled, hollow, metallic cylinder having at least one, square cut, annular end 21. A Inetallic toroid 23 of transverse diameter about three to four times the thickness of the wall and of mean diameter equal to the mean diameter of the cylinder, is provided with a circumferential groove 24 having a press fit onto the end of the cylinder 20. It will be noted that the tangent to the transverse cross section of the toroid 23 at the inner and outer intersections with the cylinder wall defines re-entrant angles of slightly more than 90.

The embodiment of FIG. 3 is similar to that of FIG. 1 except that the engagement of the toroid with the cylinder may be a slip fit. In this variation a suitable solder 27 may be flowed into the corners and between the parts to retain the same in the desired position.

Research has demonstrated that the advantages of the toroidal termination are realized to a greater extent if the major portion of the toroid lies within the cylinder, as compared to the construction of FIGS. 1 and 2 wherein the mean diameter of the toroid is essentially the mean diameter of the cylinder. Accordingly, in FIGS. 4 and 5, the toroid is provided with an annular recess 31 constituted by axially and radially extending walls conformed to receive the cylinder 20 in the manner depicted. In FIG. 4 the parts are indicated as engaged with a press fit, and in FIG. by a slip fit and solder at the joint 32. In these two examples the face 30 of the toroid is desirably parallel to the mid-plane thereof in order to meet the interior wall of the cylinder in a right angle, for reasons explained above. It will be apparent that, in the foregoing embodiments and those to be described, the soldered joint is ground and/ or polished to remove any nodes or other irregularities which may constitute zones of high electrostatic stress.

In FIGS. 6 and 7, the toroid 33 is abutted to the end of the cylinder, the CiICle about which the toroid is generated being aligned with the mean cylindrical plane of the cylinder, as indicated at P. If desired, the juxtaposed portion of the toroid may be flat to constitute an abutting fit over the whole end of the cylinder. In FIG. 6 the toroid may be welded or soldered to constitute a butt joint 34, whereas in FIG. 7 the joint is similar except for the inclusion of filleted corners 35.

FIG. 8 is essentially the same in function as the form shown, for example, in FIG. 1, except that the entire electrode, including the toroidal ends, is formed in one piece. The fillets 37 are not essential but, since fabrica tion is simplified they are shown as part of the illustrated embodiment.

The form of FIG. 9 is also of one piece construction but, for the reasons set forth with reference to FIGS. 4 and 5, the major part of the toroid is disposed interiorly of the cylinder.

FIGS. 10 to 15 illustrate toroidal ends having advantages similar to those of FIGS. 2 to 9 but which are formed of sheet metal as separate appendages or from a cylindrical blank. Particularizing, FIG. 10 shows a toroidal shell closed except for the circumferential slot 41 dimensioned to receive the cylinder 20 with a snug fit. However, to insure against the above-noted undesired effects of the sharp corners and burrs on the boundaries of the slot and the end of the cylinder it is desirable to add fillets 42 in the manner hereinabove set forth.

In FIG. 11 there is shown a variation similar in function to those forms described in connection with FIG. 4, 5 or 9. It is to be noted that in this case the toroidal end 45 is provided by inward curling of a portion of the cylinder through an angle of substantially 270 and, again, for reasons heretofore stated, the anticipated adverse effects of the edge 46 are neutralized by a fillet 47.

The modification of FIG. 12 incorporates a toroidal end 51 formed by curling the end of the cylinder through an angle of substantially 360". Here also it is desirable to include the fillet 52.

In FIG. 13 the toroidal end is constituted as a sheet metal annulus 54 which, in transverse cross section, comprises an arc of substantially 270 continued at its ends as tangential portions 54a and 54b meeting to define a recess 56 providing a seat for the end of the cylinder. The diameter of the horizontal face of the tangential portion is dimensioned to provide a snug fit within the interior of the cylinder 20. In this case also the abutting surfaces of the annulus 54 and cylinder 20 are sweated with solder.

FIG. 14 represents a variation of FIG. 13 and differs therefrom in the elimination of the tangential portion 54a and the abutment of the end of the cylinder against a matching annular surface 58 of the substantially toroidal member 59, this surface lying on the vertical mid-plane of the member. As in FIG. 13, the tangential portion 59b is a snug fit within the cylinder 20. Solder is desirably employed to close both joints between the member 59 and cylinder 20.

The embodiment of FIG. 15 is the inverse of FIG. 11 and the description of the latter is believed to be applicable to the former.

While we have shown certain embodiments of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made and we, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

We claim:

1. A lens assembly for focusing a beam of electrons on a target area comprising a first hollow cylindrical electrode, a second hollow cylindrical electrode and a third hollow cylindrical electrode, all coaxial with the principal axis of the beam and in that order, beginning at the cathode source, said first and third electrodes each being of some predetermined diameter and operated at a high potential and the second electrode being operated at a 3,309,559 5 6 substantially lower potential, said second electrode having the exterior curved surface of said second electrode is i ends Spaced apart, in an axial Sense, f the fi t and tangent to the exterior diameter of the terminations. third electrodes and having an interior diameter greater than that of both the first and third electrodes, each end References Cited by the Examiner 5 of the second electrode being provided with a toroidal UNITED STATES PATENTS termination having a transvesre diameter from two to 2903612 9/1959 Van Ormar 313 76 2,916,659 12/1959 Sage 315--5.34 four times the thickness of the Wall of said second electrode. DAVID G. REDINBAUGH, Primary Examiner.

2. The combination in accordance with claim 1 wherein 10 T, A GALLAGHER, Assistant Examiner. 

1. A LENS ASSEMBLY FOR FOCUSING A BEAM OF ELECTRONS ON A TARGET AREA COMPRISING A FIRST HOLLOW CYLINDRICAL ELECTRODE, A SECOND HOLLOW CYLINDRICAL ELECTRODE AND A THIRD HOLLOW CYLINDRICAL ELECTRODE, ALL COAXIAL WITH THE PRINCIPAL AXIS OF THE BEAM AND IN THAT ORDER, BEGINNING AT THE CATHODE SOURCE, SAID FIRST AND THIRD ELECTRODES EACH BEING OF SOME PREDETERMINED DIAMETER AND OPERATED AT A HIGH POTENTIAL AND THE SECOND ELECTRODE BEING OPERATED AT A SUBSTANTIALLY LOWER POTENTIAL, SAID SECOND ELECTRODE HAVING ITS ENDS SPACED APART, IN AN AXIAL SENSE, FROM THE FIRST AND THIRD ELECTRODES AND HAVING AN INTERIOR DIAMETER GREATER THAN THAT OF BOTH THE FIRST AND THIRD ELECTRODES, EACH END OF THE SECOND ELECTRODE BEING PROVIDED WITH A TOROIDAL TERMINATION HAVING A TRANSVERSE DIAMETER FROM TWO TO FOUR TIMES THE THICKNESS OF THE WALL OF SAID SECOND ELECTRODE. 